Monoclonal antibodies to cell surface antigens of sea urchin testicular cells will be isolated. Classes of antibodies will include those that recognize various male germ line cells but not mature sperm, those that recognize mature sperm and precursor cells, and those directed against somatic cells of the qonad. The antibodies will be used to define subpopulations of early male germ line cells (such as stem cells and spermatogonia). They will reveal differentiation antigens that will provide criteria for progression of the male germ cell lineage in vitro independent of morphology. They will also be used to interfere with germ cell- germ cell and germ cell-somatic cell interaction in culture to evaluate the roles of intercellular interactions in promoting normal spermatogenesis.